The present invention relates to apparatuses for generating acoustic waves. As used herein, the term “wave” shall include any disturbance that propagates from one point in a medium to other points without giving the medium as a whole any permanent displacement, including, but not limited to, disturbances having cyclic waveforms and disturbances having noncyclic waveforms. The term “wave” may also include pressure sequences. In any typical hydrocarbon well, damage to the surrounding formation can impede fluid flow and cause production levels to drop. While many damage mechanisms plague wells, one of the most pervasive problems is particles clogging the formation pores that usually allow hydrocarbon flow. These clogging particles can also obstruct fluid pathways in screens; preslotted, predrilled, or cemented and perforated liners; and gravel packs that may line a well. Clogging particles may even restrict fluid flow in open-hole wells. Drilling mud, drilled solid invasion, or even the porous formation medium itself may be sources for these particles. In particular, in situ fines mobilized during production can lodge themselves in the formation pores, preslotted liners, screens and gravel packs, sealing them to fluid flow. Referred to as the “skin effect,” this damage is often unavoidable and can arise at any stage in the life of a typical hydrocarbon well. The hydrocarbon production industry has thus developed well-stimulation techniques to repair affected wells or at least mitigate skin-effect damage.
The two classic stimulation techniques for formation damage, matrix acidizing and hydraulic fracturing, suffer from limitations that often make them impractical. Both techniques require the operator to pump customized fluids into the well, a process that is expensive, invasive and difficult to control. In matrix acidizing, pumps inject thousands of gallons of acid into the well to dissolve away precipitates, fines, or scale on the inside of tubulars, in the pores of a screen or gravel pack, or inside the formation. Any tool, screen, liner or casing that comes into contact with the acid must be protected from its corrosive effects. A corrosion inhibitor must be used to prevent tubulars from corrosion. Also, the acid must be removed from the well. Often, the well must also be flushed with pre- and post-acid solutions. Aside from the difficulties of determining the proper chemical composition for these fluids and pumping them down the well, the environmental costs of matrix acidizing can render the process undesirable. Screens, preslotted liners and gravel packs may also be flushed with a brine solution to remove solid particles. While this brine treatment is cheap and relatively easy to complete, it offers only a temporary and localized respite from the skin effect. Moreover, frequent flushing can damage the formation and further decrease production. In hydraulic fracturing, a customized fluid is ejected at extremely high pressure against the well bore walls to force the surrounding formation to fracture. The customized gel-based fluid contains a proppant to hold the fractures open to fluid flow. While this procedure is highly effective at overcoming near-borehole skin effects, it requires both specialized equipment and specialized fluids and therefore can be costly. Fracturing can also result in particle deposition in the formation because the gels involved may leave residue in the vicinity of the fractures.
The hydrocarbon production industry developed acoustic stimulation as an alternative to the classic stimulation techniques. In acoustic stimulation used for near-borehole cleaning, high-intensity, high-frequency acoustic waves transfer vibrational energy to the solid particles clogging formation pores. The ensuing vibrations of the solid particles loosen them from the pores. Fluid flow, including production-fluid flow out of the formation or injection-fluid flow into the formation from the well, may cause the particles to migrate out of the pores into the near-wellbore area where the greatest pressure drops exists, clearing the way for greater fluid flow. Acoustic stimulation may also be used to clean preslotted liners, screens and gravel packs. Near-well bore cleaning by acoustic stimulation has shown great promise in laboratory experiments, and the industry has developed several tools using this technique for use in real-world wells.
Acoustic stimulation tools require a compact source of acoustic waves that may be used downhole. Many current tools radiate acoustic waves over 360 degrees or in an uncontrolled direction in an attempt to reduce the skin effect along the circumference of a well bore at a given depth all at one time. These tools consume large quantities of energy to radiate waves of sufficient intensity to vibrate the solid particles along the circumference of the well bore. Supplying this energy downhole to create the necessary high-intensity acoustic waves is no easy feat, and thus these tools are poorly suited for removing solid particles from the formation. Because these tools often stretch across nearly the entire diameter of the well bore, they also cannot move through narrow passages such as production tubing or even small-diameter well bores.